Fuel Pressure Flux or Bad JPI Sensor?

[bhtitle]

Good morning,

Over the past few flights my 1967 F with JPI EDM-900 engine display has increasing fluctuations in fuel pressure indication.  Used to be steady at 23-24 almost all the time.  Now in cruising flight with boost pump off, 3000-7000 ft MSL, the pressure wanders down to 21, and up to 26 psi.  Every 5 seconds or so it dips to 21 and returns up to 24-26.

With the boost pump on it's steadier, though higher at about 27 psi.

One time after refueling a couple weeks ago, I had a red X on fuel press after hot start.  Recycled master and it showed normal.  Only saw that once.

Often following shutdown the skinny vent tube (fuel pump and intake manifold) under the left cowl makes a few drips of what looks like some combination of fuel and oil but not enough to make a puddle, just a few drips.

I've unplugged and re-plugged in the JPI sensor with no effect.  JPI system was installed May/June of 2021 and flown ~60 hours since then.

Should I be suspicious of my sensor or should I suspect something wrong with my engine driven pump?  Debris in the system?  Do you all have any suggestions or seen what a failing engine driven pump looks like?  Not sure I have access to a spare sensor without buying one for $$$ just to isolate that variable.  Engine runs and sounds normal the whole time.  This sounds like something that could cause loss of engine power if the pump fails.  Thanks!

[Browncbr1]

if it does it worse the higher you fly, then air is getting into your fuel system...  sometimes it's from the gascolator/valve selector.  Sometimes, it's from a loose fuel line fitting.   Is fuel flow unstable also?  this is usually accompanied by what feels like slight roughness or missing.  wobbly fuel pressure corroborated with wobbly fuel flow points to air in fuel.

check for loose wire connections and/or ground on for the sensor and also check that the pins are clean and straight.

clean the fuel screen on the right side of your fuel servo and also clean the gascolator and check the gasket for cracks.  Do not over torque the gascolator bowl bolt!!!  iirc 20in/lbs.  There is a little rubber coated metal washer on that bolt that should be replaced every several years.

[Ragsf15e]

9 hours ago, bhtitle said:

Good morning,

Over the past few flights my 1967 F with JPI EDM-900 engine display has increasing fluctuations in fuel pressure indication.  Used to be steady at 23-24 almost all the time.  Now in cruising flight with boost pump off, 3000-7000 ft MSL, the pressure wanders down to 21, and up to 26 psi.  Every 5 seconds or so it dips to 21 and returns up to 24-26.

With the boost pump on it's steadier, though higher at about 27 psi.

One time after refueling a couple weeks ago, I had a red X on fuel press after hot start.  Recycled master and it showed normal.  Only saw that once.

Often following shutdown the skinny vent tube (fuel pump and intake manifold) under the left cowl makes a few drips of what looks like some combination of fuel and oil but not enough to make a puddle, just a few drips.

I've unplugged and re-plugged in the JPI sensor with no effect.  JPI system was installed May/June of 2021 and flown ~60 hours since then.

Should I be suspicious of my sensor or should I suspect something wrong with my engine driven pump?  Debris in the system?  Do you all have any suggestions or seen what a failing engine driven pump looks like?  Not sure I have access to a spare sensor without buying one for $$$ just to isolate that variable.  Engine runs and sounds normal the whole time.  This sounds like something that could cause loss of engine power if the pump fails.  Thanks!

So as @Browncbr1 alluded to, there are lots of things that can cause this. I would say it’s unlikely that it’s the sensor. I have an EDM 930 and had the exact same indication for about two years.  JP I was adamant that the sensors don’t normally fail that way. They did say it could be an EMI off of an unshielded wire in the engine compartment. However I had steady indications a few years back, then fluctuating for ~2 years, now steady again.  Mine was finally fixed by re-securing all the fuel lines. I think I had a very very small air leak somewhere. When I finally got a steady indication again, it was after I removed and reinstalled all the connections from the fuel servo to the fuel pressure sensor. I’m not saying that fixed it, but for the last six months or so I’ve had a nice steady fuel pressure indication. I’ll try to attach the thread where we went through all of this a few months back.

To answer your question though, a failing mechanical fuel pump will leak out the drain in the bottom of the engine compartment. It will be either oil or fuel coming out. There are two diaphragms in there. If one fails it should still work and it won’t fluctuate. If both fail, well that’s not good.  It should not cause a fluctuation.

[Ragsf15e]

This thread is good…

 

[Browncbr1]

I used permatex thread sealant on the tightening NPT fittings on my fuel flow transducer to solve mine a few years back.... then had the same issue, but less pronounced and it was solved by rebuilding the fuel selector with all new o-rings.  I saw the slightest hint of dye on the FF transducer, but none on the selector.  

Hopefully, there is no leak coming from any of the NPT fittings on your mechanical fuel pump.  it's very tight access back there.

[Ragsf15e]

Yes, I called tempest to ask about the fuel pump causing this, and they said that was unlikely, but they do have problems with the O-rings on the connection into the fuel pump. So that’s another place to look. You may not see fuel leaking it may just be air leaking very small into the system. You’ve got to have sealed fuel fittings.

[Ragsf15e]

5 minutes ago, Browncbr1 said:

I used permatex thread sealant on the tightening NPT fittings on my fuel flow transducer to solve mine a few years back.... then had the same issue, but less pronounced and it was solved by rebuilding the fuel selector with all new o-rings.  I saw the slightest hint of dye on the FF transducer, but none on the selector.  

Hopefully, there is no leak coming from any of the NPT fittings on your mechanical fuel pump.  it's very tight access back there.

I agree with having the sealed fittings, unfortunately most of the fittings are flared fittings and aren’t supposed to require sealant. I think even a small air leak can cause this.

[Browncbr1]

1 hour ago, Ragsf15e said:

I agree with having the sealed fittings, unfortunately most of the fittings are flared fittings and aren’t supposed to require sealant. I think even a small air leak can cause this.

Right, I’m just talking about the expanding threads that go into the aluminum transducer housing.   The jpi housing is easy to crack after just a few threads.  Putting thread sealant on flare fittings would be useless. Haha

[bhtitle]

Thank you all for the detailed info and the linked thread.  Fuel flow is steady, or at least doesn't wander more than +/- 0.2 gal/hr, which is how it has always been since JPI installation.  I'll search for air leaks first, clean the fuel servo screen and gascolator (the stamp on mine says 25 in-lbs), and will look carefully when I fly next whether the behavior is sensitive to altitude.  Also fuel selector o-rings - mine don't leak externally but I would be surprised if those or the fuel bowl bolt washer have been changed in years.

Much appreciated!

[Ragsf15e]

4 minutes ago, bhtitle said:

Thank you all for the detailed info and the linked thread.  Fuel flow is steady, or at least doesn't wander more than +/- 0.2 gal/hr, which is how it has always been since JPI installation.  I'll search for air leaks first, clean the fuel servo screen and gascolator (the stamp on mine says 25 in-lbs), and will look carefully when I fly next whether the behavior is sensitive to altitude.  Also fuel selector o-rings - mine don't leak externally but I would be surprised if those or the fuel bowl bolt washer have been changed in years.

Much appreciated!

No problem.  It’s a frustrating issue because the air leaks (if that’s the cause) are invisible.  
Be careful and be sure about the torque on the servo bowl.  It’s breakable and hard to replace.  The spec is in the maintenance manual.  Just be gentle and it’s ok.  Good luck!